Emery (mineral)

Emery is a very hard rock type used to make abrasive powder. It largely consists of the mineral corundum (aluminum oxide), mixed with other species such as the iron-bearing spinels hercynite and magnetite, and also rutile (titania). Industrial emery may contain a variety of other minerals and synthetic compounds such as magnesia, mullite, and silica.

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It is black or dark gray in colour, less dense than translucent-brown corundum with a specific gravity of between 3.5 and 3.8. Because it can be a mix of minerals, no definite Mohs hardness can be assigned: the hardness of corundum is 9 and that of some spinel-group minerals is near 8, but the hardness of others such as magnetite is near 6.

Crushed or naturally eroded emery (known as black sand) is used as an abrasive — for example, on an emery board, as a traction enhancer in asphalt and tarmac mixtures, or as used in mechanical engineering as Emery cloth.

The Greek island of Naxos used to be the main source of this industrially important rock type. It has been mined on the eastern side of Naxos for well over two thousand years until recent times. However, demand for emery has decreased with the development of sintered carbide and oxide materials as abrasives.